Journal of Chromatography, 130 (1977) 4&+%59 8 Elsevier Scientific Publishing Company, Amsterdam - Printed ir. The Netherlands

CHROM. 9500

Letter to the Editor

The compressibility effects in syringe pumps for liquid chromatography

Sir, Achener et 01.’ present a refinement of our study of the effects of solvent

compressibility in syringe pumps used for liquid chromatographyZ by taking into ac- count the variation in soIvent compressibility with pressure; we wish to make two comments on that paper.

First, we are p!eased to see that the conclusions of this second-order approach agree qualitatively with those of our first-degree approximation, although, naturally, the quantitative agreement between calculated and experimental data becomes closer when using the equation derived by Achener et al.

We certainly agree, as mentioned previously’, that compressibilities at atmo- spheric pressure cannot be extrapolated to high pressures without introducing sig- nificant errors, but it should be emphasized that n-pentane is an extreme case, the one chromatographic solvent for which both the compressibility, x, and d;CldP are most important: hence the difference between the predictions of the first- and second-order assumptions might be expected to be small in practice -less than 20% at 100 atm.

For completeness, however, we wish to mention that the expression governing variation in compressibility with pressure used by Achener et al. is a first approxima- tion of the Tait equation3 and that an exact treatment of the problem would necessitate taking account of the variation in soIvent viscosity with pressure, which can have important effects3.

Finally, our simpiified approach gives reasonably good results, even at high pressures, if an average compressibility is used: this is important, because very few accurate data exist in the literature on the coefficients of the Tait equation, even data on the compressibilities of liquids are hard to find, and such data vary largely from one author to another’.‘.

These minor points should obscure neither the basic agreement between us and Achener et al. that a finite time is needed to achieve steady-state conditions of pressure and fiow-rate (and that this time depends on the column characteristics, on the solvent and on the volume of the reservoir), nor our agreement on the order of magnitude of this time.

We are also glad to note that manufacturers have studied these problems and can offer some means for obtaining satisfactory results with syringe pumps under analytical conditions_ One solution is to use, momentarily, a very high pumping speed, unti1 the pressure corresponding to the desired steady-state flow-rate is attained. This, of course, requires that the pressure be known from previous measurements, which is usually so in routine analysis only. For this purpose, however, the pump motor must be quite powerful, and not all manufacturers offer pumps with aspeed sufficiently high for this operation to be practical_ Similarly, the good results described by Achener et al--in their study of stop-flow injection can be obtained only if the valve is close

LETTER TO THE EDITOR 4.59

to a low-volume injection port. It should be emphasized that, in both cases, the solu- tions suggested by Achener et al. do not seem to be easily applicable to preparative liquid chromatography, to say the least.

In practice, the judicious use of these refinements is possible only if the phenomenon to be minimised (and, perhaps, overcome) is we11 known and under- stood. That. was the aim of our original paper; we are happy that it seems to have been achieved.

l%oie Polytechnique, Route de Saclay, MICHEL MARTIN

91120 Palaiseau (France) _ GEORGES GUIOCHON

ELF-ERAP, DtGgation ci la Recherche,

7, rue Ndaton, Paris (France) GILBERT BLU

Department of Chemistry, Northeastern University, Boston, Mass. (U.S. A.)

CLAUDE EON

1 P. Achener, S. R. Abbott and R. L. Stevenson, J. Chromatogr., 130 (1977) 29. 2 M. Martin, G. Blu, C. Eon and G. Guiochon. J_ Chromarogr., 112 (1975) 399. 3 M. Martin, G. Blu and G. Guiochon, J. Chronmtogr. Sci., 11 (1973) 641.

(Received July 6th, 1976)

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